The human immune response genes are located in the D region of the major histocompatibility complex (MHC). They encode the alpha and beta chains of the HLA-DR, -DQ and -DP class II antigens. The aim of this project is to define by a molecular approach how many functional class II genes exist and to express individual class II antigens in cell lines by DNA-mediated gene transfer in order to analyze the interaction between class II antigens and T lymphocytes. cDNA clones in eukaryotic expression vectors have been obtained for the alpha and beta chains of the DP, DQ and DR antigens. Various murine, simian and human cell lines have been transfected with these clones and express defined human class II antigens at their surface. This expression system has allowed us to show that the invariant chain, a glycoprotein associated intracellularly with class II atigens, was not necessary for cell surface expression of class II antigens. It was also possible to show that the DR antigens interact with the 4 molecules, a cell surface antigen expressed by a subset of T lymphocytes. Transfected cell lines expressing class II antigens are able to present foreign viral antigens to T lymphocytes with appropriate specificity for viral antigens and MHC molecules. This system can be used to define the elements important for antigen-recognition by T lymphocyes. New class II genes have been identified and isolated as complete cDNA clones; they are called DO beta and DZ alpha. Whereas DZ alpha is expressed like a typical class II gene in B cells, activated T cells and cells treated with gamma interferon, DO beta expression is restricted to B cells. By use of pulsed-field gel electrophoresis it was possible to determine the complete linkage of class II genes at the molecular level and to precisely map the DO beta and DZ alpha genes within the 1100 kilobases present in the D region of HLA. The availability of complete cDNA clones for DO beta and DZ alpha makes it possible to analyze these putative new class II antigens at the protein level.